How do i know what you need !!!!
You have to look at the level of the hero and skill required for the exchange. I haven't seen an exchange that requires blue or purple essence, but I can check when I go ingame.
Sorry for the late reply, just leaving game. Simply click on the + sign and it will show you the hero, required level, skill and talent required for the exchange. Blue nor Purple essence are required.
I talked to the support guys and i know how.
You need to have the required hero lvl and skill plus you must remove the hero from any tower andunlock it if you have it locked . Remove the pets from the hero . Hero should not be in tourch or Narncia . Plus remove the insignia